It is known that, in a Radio Access Network (RAN), such as a Universal Mobile Telecommunications System (UMTS) RAN, the radio base station has two basic subsystems, namely the Radio Equipment Control (REC) and the Radio Equipment (RE). The Common Public Radio Interface (CPRI) defines a specification for the internal interface of radio base stations between the Radio Equipment Control (REC) and the Radio Equipment (RE).
In some circumstances, the particular Radio Access Network might require that the nodes of the network should be synchronized, to within a specified time. In such a case, it will often be necessary to calibrate for the delay introduced by the link between the nodes, in particular when the Radio Equipment Control and the Radio Equipment are not co-located.
In many implementations, the link between the Radio Equipment Control (REC) and the Radio Equipment (RE) uses a point-to-point optical fibre connection. In that case, it can be assumed, with an acceptable degree of accuracy, that the connection is symmetric, that is, that the uplink delay is equal to the downlink delay. The round-trip delay, that is, the time taken for a signal to pass from a first node to a second node, and then back to the first node, can be measured accurately. Therefore, if it can be assumed that the uplink delay is equal to the downlink delay, the one way delay can be estimated from the round trip delay with acceptable accuracy.
However, when the link uses an asymmetric connection, in which the uplink delay is not necessarily equal to the downlink delay, the one way delay cannot be estimated from the round trip delay with the degree of accuracy that is required.